During the same time period, the higher frequency SF dialect increased from 29% to 95%.

"In response to high levels of low-frequency ambient noise, urban birds have songs with higher frequencies," the authors write.

Kaplan says the study not only found the lower frequency dialect P became extinct in the region, but that the minimum frequency of the remaining dialects LM and SF increased compared to their rural counterparts.

She says the current theory on how birds and other animals communicate suggests they try to talk to each other at a frequency level that cuts through the ambient noise.

According to Kaplan, most studies have found bird dialects are consistent and stabilise over generations.

Environmental push

If there are no changes in the environment there is no reason for the songs to change, says Kaplan.

But in the case of white-crowned sparrows in urban areas, song changes occurred between generations, rather than within a generation.

This is mainly due to their short two-year lifespan, she says.

"Bird species with short lifespans need to find a partner to mate with quickly, in many cases they get tutors to teach them the songs."

Kaplan says the study is a great paper because: "It's one of the very few papers on bird songs done over a long period of time."

She says it is a great example of what Darwin was talking about: "Those who adapt survive."

"This study shows quite clearly that they've adapted because they're still mating well and communication patterns haven't been affected."

But Kaplan says this study should be a wake-up call.

If it can happen to one species in one urbanised area, it could happen to many species, she says.